This invention relates to mobile cranes or aerial lift platforms. In each case a turntable or its equivalent is provided rotatably mounted on the chassis of the vehicle and carrying a boom or jib which may be telescopic and which carries the hoist load suspension cables or the man carrying platform or basket.
Since a forward overturning moment is created by the weight of the boom together with the load lifted in the case of a crane or with the basket and operator of the lifting platform, it is necessary to introduce a counter-balancing or stability moment by fitting some form of ballast weight at the most advantageous part of the rotating structure.
On many machines unless they are fitted with suitable outriggers or extending axles, the overturning condition is most unfavourable when the rotating structure is at 90.degree. to the longitudinal axis of the chassis. In this position it is also essential to ensure that the rearward stability condition of the machine, i.e. with the boom head point at its minimum radius, is satisfactory.
To meet these stability requirements and to achieve the most efficient and economical use of material it has hitherto been necessary to fit an additional counterweight to the chassis normally in the form of separate cast iron or steel weights bolted to the chassis frame structure, the frame being constructed from rolled steel sections, i.e. joist, channel or plate fabrication.
This results in a chassis unit which is expensive to fabricate and assemble. It is further complicated by the need to provide a stiff base on the chassis frame to support the rotating upper machine structure.